Transfer membrane apparatus

ABSTRACT

A transfer membrane assembly in which one fluid is passed with a pulsatile flow through a first conduit between two transfer membranes which are provided with a close packed array of dimples so that vortex mixing occurs. Second conduits are formed on the other side of each membrane between that membrane and an adjacent profiled plate. Each dimple partially nests in a depression in the respective plate, the depressions being connected in rows by grooves. The maximum diameter and depth of each dimple is less than the corresponding dimensions of the respective depression.

DESCRIPTION

My EP-A-0111423 discloses a membrane assembly (hereinafter referred toas of the kind described) for use in mass or heat transfer apparatus,and comprising a pair of generally parallel plates spaced apart with aprofiled surface of one plate facing a similarly profiled surface of theother plate, and a pair of sheets of transfer membrane material closelyspaced, throughout substantially their whole area, face to face betweenthe plates with each sheet supported by and partially nesting in theprofiled surface of the adjacent plate, thereby providing between themembranes a first conduit for a first fluid, and between each membraneand the adjacent plate a second conduit for a second fluid, eachmembrane presenting in its surface facing the other membrane a regularclose packed array of permanent dimple-like depressions each facing andsubstantially aligned with a similar depression in the other membrane,and the profiled surfaces of the plates being provided by depressions inalignment with the dimples in the respective membrane and interconnectedby grooves; the arrangement being such that if, in use, one fluid ispassed through the first conduit with a pulsatile flow, vortex mixingoccurs in the dimples in the membranes.

The pulsatile flow may be produced by superimposing on a mean flow ofthe first fluid through the first conduit a pulsatile component whichpromotes the vortex mixing of the fluid. As particularly described inthe EP specification, the dimples in each membrane are provided inparallel rows the dimples in each row being longitudinally offsethalfway between adjacent dimples in the adjacent rows so that lateralnesting of adjacent rows occurs. The depressions in the plates are alsoarranged in longitudinal rows along the direction of the mean flow, thegrooves interconnecting one depression to the next along the respectiverow.

In this kind of membrane assembly, the efficiency of the transfer acrossthe membrane is high as the total volume of first fluid which can bebrought into close proximity with the membrane, as a result of thevortex mixing, is high.

Apparatus using such a transfer membrane assembly has been usedsuccessfully for the oxygenation of blood, in which case blood is passedthrough the first conduit and oxygen-containing gas through the secondconduits. In this case the membrane have usually been hydrophobic. Theassembly has also been used successfully for filtration of aqueousliquid, in which case the membranes are preferably hydrophilic, so thatgood wetting occurs. For example, filters incorporating the membraneassembly have proved extremely successful for harvesting plasma fromwhole blood by plasmaphersis or using donated blood, as the assemblyprovides high filtration rates per unit membrane area, works well at lowrates of blood flow (60-80 ml/min) and is not prone to the kind offouling which is a complication of conventional membrane plasma filters.In fact filtration rate has been about four times as high as thatachieved by conventional membrane filters of the same area. Also,filters incorporating such a membrane assembly have proved successfulexperimentally in the concentration by filtration of bacterial,mammalian or plant cells in biotechnological work. In these cases thewhole blood or other liquid to be filtered is passed through the firstconduit and the filtrate is collected in the second conduits.

Each second conduit will have an inlet and an outlet if through flow isrequired, e.g. in countercurrent to the flow of the first fluid throughthe first conduit. On the other hand, if the apparatus is set up forfiltration, only an outlet from each second conduit may suffice.

In spite of these successes with the membrane assembly described in theEP specification, there are two areas where I have sought improvement.

The first is that, as clearly illustrated in FIG. 5 of the EPspecification, the maximum diameter of each dimple in each membrane wasthe same as that of the associated depression in the adjacent profiledsupport plate. As a result, there was a lower limit to the radius ofcurvature of dimple to maintain adequate spacing between the membraneand the wall of the corresponding depression in the plate, in order toavoid undue restriction of the corresponding second conduit and theproduction of areas of stagnation at the periphery of the zone definedbetween each dimple and the corresponding depression in the plate. Thisin turn dictated a minimum acceptable aspect ratio of maximum diameterto maximum depth of each dimple of 3:1 as mentioned in the EPspecification, but in practice of 5:1. As a result the dimples havepresented to the first conduit a shallowness which has been found todeter the promotion in the dimples of the vortices which are critical tothis kind of membrane assembly.

Second, when the dimples in the membranes are preformed, and since thedimensions of the depressions in the profiled plates are of the order ofmagnitude of 1 mm, I have found great difficulty in accurately locatingthe membranes relatively to the profiled plates so that the dimples arecentred in the plate depressions, during assembly of the plates andmembranes. Any misalignment distorts the membranes and seriouslyinterferes with the predetermined flow patterns on both sides of themembranes, and hence leads to a critical loss of efficiency.Furthermore, when the membranes are hydrophilic, even if they are formedin situ by the pressure of hot liquid in the first conduit, as describedin the EP specification, they swell and their dimples readily becomedislodged from the corresponding depressions in the profiled plates.

According to one aspect of the present invention, in a membrane assemblyof the kind described, the maximum external diameter of each dimple, ina plane substantially parallel to the plane of the correspondingmembrane sheet, is less than the corresponding diameter of thecorresponding plate depression. The dimple diameter is preferably atleast 10% less than, and most preferably substantially 20% less than,that of the plate depression.

Although it might be expected that this reduction in the maximumdiameter of each dimple, would inhibit the production of vortices in thefirst fluid, the contrary appears to be the case. This apparentlyresults from the increased angle which the peripheral edge of eachdimple can make with the plane of the corresponding membrane sheetand/or in a reduction in the aspect ratio of the maximum dimple diameterto maximum dimple depth, which is preferably now less than 3. As aresult, I have found that a filter incorporating a membrane assemblyaccording to the invention can provide, when carrying outplasmafiltration, and as compared with a filter utilising a membraneassembly in accordance with the earlier EP specification, an increase inthe plasma filtration rate from 18 ml/min using 200 cm² of membrane, to35 ml/min for the same membrane area, i.e. an improvement ofsubstantially 100%. The improved vortex mixing within the dimples has,when the assembly is used in a plasma filter, apparently also resultedin a reduction in fouling of the membrane by blood proteins and cells,no doubt as a result of the additional shear effect adjacent to themembrane within the dimples. Furthermore, when the membrane sheets arepreformed with the dimples, because the dimples have a smaller maximumdiameter than the corresponding depressions in the plates, assembly isremarkably simpler and the membranes stay located in the correctposition relatively to the support plates each whey they have beenthoroughly wetted and have swelled up.

Suitable hydrophilic microporous membranes for plasmafiltration or forthe concentration of bio-cells are made from polysulphone and have poresof the order of 0.2 μ to 0.45 μ in diameter. Such membranes may bepreformed with the appropriately shaped dimples by pressing them at atemperature of 80° C. between a female former made of brass and a maleformer moulded from silicone rubber. The male projections on the maleformer may consist of a hemispherical end portion supported by acylindrical pedestal of the same diameter.

As distinct from the U-shaped conduits illustrated in the earlier EPspecification, it is now considered to be simpler if the conduits arelinear, for example if the plates are rectangular and an inlet and anoutlet for the first fluid are provided adjacent to opposite ends of theplates. Manifolds may be provided at the opposite ends of the opposedrectangular plates for the first conduit, and the pulsatile flow throughthe first conduit is preferably provided by diaphragm pumps withflexible diaphragms extending across the ends of the plates and definingwalls of the first conduit manifolds.

A further consideration with membrane assemblies of the kind describedis that when they are first primed before use, particularly when thesecond fluid is a liquid, difficulties arise in clearing all residualair from the depressions and grooves constituting the second conduits.The earlier EP specification discloses the possible provision ofadditional grooves extending transversely across the rows of depressionsin the profiled plates to interconnect the longitudinal grooves to helpprevent stagnation along a whole row resulting from blockage of a signaldepression in that row. This does not significantly alleviate theproblem of purging any residual air upon priming.

According to a second, independent, aspect of the invention, but whichmay be used in combination with the first aspect of the invention, in amembrane assembly of the kind described, the profiled surface of eachplate is additionally provided with a network of interconnectingchannels which intersect the grooves and lead to an outlet for thesecond conduit, each channel having a larger cross section than that ofa groove. This provides overall irrigation of the second conduits whichboth avoids areas of stagnation and also assists in purging air when theassembly is primed with a working liquid.

In the preferred case in which the plates are rectangular, the channelspreferably extend one along each longer side of each plate, with aplurality of transverse channels interconnecting the side channels, toprovide a ladder-like array of channels. The spacing of the transversechannels may be at least the spacing of every tenth depression in a row.

The problem of clearing all residual air from the depressions andgrooves constituting the second conduits is also exacerbated if themembrane assembly is used in an orientation in which the grooves extendhorizontally. It is then difficult for the air bubbles to rise within amatrix of liquid. It is therefore considered desirable to use theassembly with the grooves extending vertically, or at least with avertical component. This conveniently involves using the assembly withthe faces of the plates, and the general planes of the membrane, invertical planes. However, if the plates are rectangular and the grooves,and hence the interconnected rows of depressions, extend parallel to thelength of the plates, as might be through most natural for conveyence ofthe second fluid towards an outlet adjacent to an end of the plates, orin countercurrent to the first fluid, it would not be possible to mountthe assembly with the length of the plates horizontal, which is the mostdesired configuration to balance the pump action at each end of thefirst conduit.

According to a third, independent, aspect of the invention, but whichmay be used in combination with the first and/or second aspect(s) of theinvention, in a membrane assembly of the kind described, each plate issubstantially rectangular and the first conduit is arranged to directthe flow therethrough from one end to the other along the length of theplates, the grooves, and hence the interconnected rows of depressionsextending at an inclination to the length of the plates.

When the assembly is used with a longer edge of the plates lowermost,the inclined nature of the grooves, e.g. at 90°, or obliquely, to theedges of the plates, encourages any residual air bubbles to rise to theupper edge of the second conduits whilst, if the inclination is oblique,also promoting transportation along the plates towards an outlet fromthe respective second conduit. Outlets from the second conduits willthen be provided adjacent to the upper edge of the second conduits, atone end if through flow of the second fluid is required, or at anyconvenient position if only an outlet is required. The profiled surfaceof each plate may be provided with a channel which extends along theuppermost longer side of the plate, intersects the grooves and leads toan outlet for the second conduit, the channel having a larger crosssection than that of a groove.

An example of apparatus incorporating a membrane assembly constructed inaccordance with the present invention, is illustrated in theaccompanying drawings, in which:

FIG. 1 is a diagrammatic perspective view of the apparatus;

FIG. 2 is a section taken on the line II--II in FIG. 1;

FIG. 3 is a section taken on the line III--III in FIG. 1;

FIG. 4 is a section taken on the line IV-IV in FIG. 1;

FIG. 5 is a plan of the profiled surface of one plate of the reactor;and,

FIGS. 6 and 7 are sections through the two plates of the reactorjuxtaposed with two membranes between them, and taken respectively onthe lines VI--VI, and VII--VII, in FIG. 5;

As shown in FIG. 1, the apparatus 13 is carried face to face by anupright wall 14 and consists of similar opposed side plates 15 andsimilar pairs of end plates 16 and 17. The side plates 15 arerectangular and elongate and the facing adjacent surfaces of theseplates are profiled. Positioned between the two profiled surfaces of theside plates 15 are a pair of membranes 18. Along the longer sides of theplates 15, the membranes 18 are sealed to one another and to the plates15 by clamping bolts 19, which draw the plates together, and pairs ofsealing beads 20, which are seated in grooves in the plates 15, and abutthe the membranes 18. There is thus formed between the membranes 18, acentral first conduit 22 and, between each membrane 18 and the adjacentprofiled surface of the adjacent plate 15, and outer second conduit 23.The profiled surfaces of the plates are also formed with a ladder-likearray of channels 24 which ensure complete irrigation and initialdebubbling of the second conduits 23 between the membranes and profiledfaces of the plates 15.

At each of the ends of the plates 15, the two plates 16 and 17 arebolted to them by bolts 25 and the ends of the membranes 18 are clampedbetween the ends of the plates 15 and the end plates 16. Clamped betweeneach of the plates 16 and the adjacent plate 17, is it an outwardlyextending flange 26 of a flexible diaphragm 27. A manifold 28, incommunication with the adjacent end of the primary chamber 22, is formedwithin an open interior of the plate 16 and each of these manifolds 28is connected through a bore 21 with an external nipple and hose 29, 29'.The diaphragm 27 are accommodated within openings within the respectiveplates 17 and are acted upon by respective pushers 30, 30' carried byarms 31, 31', which work through elongate slots 32, 32' in the board 14,and carried on respective ends of a member 33. This member isreciprocable in a linear bearing 34 by means of a motor 35 actingthrough a crank 36. As the member 33 is moved to and fro liquid isflushed to and fro through the first conduit 22. However, if mean flowthrough the conduit 22 is required, the stroke of the pusher 30 extendsfurther into the respective plate 17 than does the pusher 30', as aresult of which there is superimposed upon the reciprocation flow in thefirst conduit 22, a component which provides a net mean flow from theinlet hose 29 to the outlet hose 29'.

At each end of the plates 15, each of the second conduits 23 and arespective one of the channels 24 communicates through a port 37 in therespective plate via a bore 38 in the respective plate, with a nippleand hose 39, 39'.

As suggested in FIGS. 2, 3 and 4, shown more clearly in the enlargedFIGS. 5, 6 and 7, the profiled face of each plate 15 is provided with aclose packed array of substantially hemispherical depressions 40 whichare arranged in parallel rows extending obliquely at 60° to the longerdimension of the plate, depressions in each row being offset half waybetween those in adjacent rows, to provide the close packing. Adjacentdepressions in each row are interconnected by grooves 41. The membranes18 are each performed with an array of dimples 42, centred on thedepressions in the plates such that when the membranes are assembledbetween the plates, the dimples 42 partially nest in the depressions, asshown in FIGS. 6 and 7. The first conduit 22 is thus formed in thecontinual spacing between the membranes 18, and the second conduits 23are formed by the spaces between the bottoms of the dimples and thebottoms of the corresponding depressions, together with the grooves 41.Each depression 40 has a maximum diameter of 1.5 mm, and a maximum depthof 1.3 mm and each dimple 42 has a maximum external diameter of 1.2 mmand a maximum depth of 0.42 mm. The maximum external diameter of adimple 42 is therefore 20% less than that of a depression 40 and theaspect ratio of the maximum diameter to the maximum depth of each dimpleis 2.85.

When the dimensions of the depressions and dimples are as above, itwould be appropriate for the transverse channels 24 to be spaced atabout 20 mm intervals along the plates. To appreciate what this means inthe context, each of the plates 15 is substantially 150 mm long and 100mm wide. When the apparatus is used in the orientation shown in FIG. 1,the lowermost horizontal channel 24 could be omitted. Also, if thegrooves 41 were inclined at 90° to the length of the plates, i.e.vertically, the vertical channels 24 could also be omitted, leaving onlythe horizontal channel 24 along the uppermost longer edge of each plate.

In use when liquid in the first conduit 22 is flushed to and for betweenthe manifolds 28, by the out of phase action of the pushers 30, vorticesin the liquid are set up in the dimples 42. This brings a greaterquantity of the liquid into intimate contact with the membranes andhence enhances the transfer of gas or other material through themembranes to or from the liquid.

I claim:
 1. A membrane assembly for use in mass or heat transferapparatus and comprising a pair of generally parallel plates spacedapart with a profiled surface of one plate facing a similarly profiledsurface of the other plate, and a pair of sheets of transfer membranematerial closely spaced, throughout substantially their whole area, faceto face between the plates with each sheet supported by and partiallynesting in the profiled surface of the adjacent plate, thereby providingbetween the membranes a first conduit for a first fluid, and betweeneach membrane and the adjacent plate a second conduit for a secondfluid, each membrane presenting in its surface facing the other membranea regular close packed array of permanent dimple-like depressions eachfacing and substantially aligned with a similar depression in the othermembrane, and the profile surfaces of the plates being provided bydepressions in alignment with the dimples in the respective membrane andinterconnected by grooves; the arrangement being such that if, in use,one fluid is passed through the first conduit with a pulsatile flow,vortex mixing occurs in the dimples in the membranes; characterized inthat the maximum external diameter of each dimple, in a planesubstantially parallel to the plane of the corresponding membrane sheet,is at least 10% less than that of the corresponding plate depression. 2.An assembly according to claim 1, in which the maximum diameter of eachdimple is substantially 20% less than that of the corresponding platedepression.
 3. An assembly according to claim 2, in which the aspectratio of the maximum diameter of each dimple to the maximum depth of thedimple is less than
 3. 4. An assembly according to claim 1, in which theaspect ratio of the maximum diameter of each dimple to the maximum depthof the dimple is less than
 3. 5. An assembly according to claim 1, inwhich the plates are rectangular and an inlet and an outlet for thefirst fluid are provided adjacent to opposite ends of the plates.
 6. Anassembly according to claim 5, in which manifolds are provided at theopposite ends of the opposed rectangular plates for the first conduit,and there are diaphragm pumps with flexible diaphragms extending acrossthe ends of the plates and defining walls of the first conduitmanifolds, in use to provide the pulsatile flow through the firstconduit.
 7. An assembly according to claim 5, which is arranged to beused with the faces of the plates in vertical planes and with the lengthof the plates substantially horizontal, and in which the grooves, andhence the interconnecting rows of depressions extend at an inclinationto the length of the plate.
 8. An assembly according to claim 1, inwhich the dimples in each membrane are provided in parallel rows withthe dimples in each row being longitudinally offset halfway betweenadjacent dimples in the adjacent rows so that lateral nesting ofadjacent rows occurs, the depressions in the plates also being arrangedin corresponding rows with the grooves interconnecting one depression tothe next along the respective row.
 9. An assembly according to claim 8,in which the profiled surface of each plate is provided with a channelwhich extends along the uppermost longer side of the plate, intersectsthe grooves and leads to an outlet for the second conduit, the channelhaving a larger cross section than that of a groove.
 10. An assemblyaccording to claim 1, in which the aspect ratio of the maximum diameterof each dimple to the maximum depth of the dimple is less than
 3. 11. Anassembly according to claim 1, in which the plates are rectangular andan inlet and an outlet for the first fluid are provided adjacent toopposite ends of the plates.
 12. A membrane assembly for use in mass orheat transfer apparatus and comprising a pair of generally parallelplates spaced apart with a profiled surface of one plate facing asimilarly profiled surface of the other plate, and a pair of sheets oftransfer membrane material closely spaced, throughout substantiallytheir whole area, face to face between the plates with each sheetsupported by and partially nesting in the profiled surface of theadjacent plate, thereby providing between the membranes a first conduitfor a first fluid, and between each membrane and the adjacent plate asecond conduit for a second fluid, each membrane presenting in itssurface facing the other membrane a regular close packed array ofpermanent dimple-like depressions each facing and substantially alignedwith a similar depression in the other membrane, and the profiledsurfaces of the plates being provided by depressions in alignment withthe dimples in the respective membrane and interconnected by grooves andadditionally provided with a network of interconnecting channels whichintersect the grooves and lead to an outlet for the second conduit, eachchannel having a larger cross section than that of a groove; thearrangement being such that if, in use, one fluid is passed through thefirst conduit with a pulsatile flow, vortex mixing occurs in the dimplesin the membranes; characterized in that the maximum external diameter ofeach dimple, in a plane substantially parallel to the plane of thecorresponding membrane sheet, is less than the corresponding diameter ofthe corresponding plate depression.
 13. A membrane assembly for use in amass or heat transfer apparatus and comprising a pair of generallyparallel plates spaced apart with a profiled surface of one plate facinga similarly profiled surface of the other plate, and a pair of sheets oftransfer membrane material closely spaced throughout substantially theirwhole area, face to face between the plates with each sheet supported byand partially nesting in the profiled surface of the adjacent plate,thereby providing between the membranes a first conduit for a firstfluid, and between each membrane and the adjacent plate a second conduitfor a second fluid, each membrane presenting in its surface facing theother membrane a regular close packed array of permanent dimple-likedepressions each facing and substantially aligned with a similardepression in the other membrane, and the profiled surfaces of theplates being provided by depressions in alignment with the dimples inthe respective membrane and interconnected by grooves; the arrangementbeing such that if, in use, one fluid is passed through the firstconduit with a pulsatile flow, vortex mixing occurs in the dimples inthe membranes, characterized in that the profiled surface of each plateis additionally provided with a network of interconnecting channelswhich intersect the grooves and lead to an outlet for the secondconduit, each channel having a larger cross section than that of agroove.
 14. A membrane assembly for use in mass or heat transferapparatus and comprising a pair of generally rectangular parallel platesspaced apart with a profiled surface of one plate facing a similarlyprofiled surface of the other plate, and a pair of sheets of transfermembrane material closely spaced throughout substantially their wholearea, face to face between the plates with each sheet supported by andpartially nesting in the profiled surface of the adjacent plate, therebyproviding between the membranes a first conduit for a first fluid, aninlet and outlet for the first fluid adjacent to opposite ends of theplates, and between each membrane and the adjacent plate a secondconduit for a second fluid, each membrane presenting in its surfacefacing the other membrane a regular close packed array of permanentdimple-like depressions each facing and substantially aligned with asimilar depression in the other membrane, and the profiled surfaces ofthe plates being provided by depressions in alignment with the dimplesin the respective membrane and interconnected by grooves andadditionally provided with a network of interconnecting channels whichintersect the grooves and lead to an outlet for the second conduit, eachchannel having a larger cross section than that of a groove; thearrangement being such that if, in use, on fluid is passed through thefirst conduit with a pulsatile flow, vortex mixing occurs in the dimplesin the membranes; characterized in that the maximum external diameter ofeach dimple, in a plane substantially parallel to the plane of thecorresponding membrane sheet, is less than the corresponding diameter ofthe corresponding plate depression.
 15. An assembly according to claim14, in which the channels extend one along each longer side of eachplate, with a plurality of transverse channels interconnecting the sidechannels, to provide a ladder-like array of channels.
 16. A membraneassembly for use in mass or heat transfer apparatus and comprising apair of generally rectangular parallel plates spaced apart with aprofiled surface of one plate facing a similarly profiled surface of theother plate, and a pair of sheets of transfer membrane material closelyspaced, throughout substantially their whole area, face to face betweenthe plates with each sheet supported by and partially nesting in theprofiled surface of the adjacent plate, thereby providing between themembranes a first conduit for a first fluid, manifolds attached at theopposite ends of the opposed rectangular plates for the first conduit,diaphragm pumps with flexible diaphragms extending across the ends ofthe plates defining walls of the first conduit manifolds, in use toprovide a pulsatile flow through the first conduit an inlet and anoutlet for the first fluid adjacent to opposite ends of the plates, andbetween each membrane and the adjacent plate a second conduit for asecond fluid, each membrane presenting in its surface facing the othermembrane a regular close packed array of permanent dimple-likedepressions each facing and substantially aligned with a similardepression in the other membrane, and the profiled surfaces of theplates being provided by depressions in alignment with the dimples inthe respective membrane and interconnected by grooves and additionallyprovided with a network of interconnecting channels which intersect thegrooves and lead to an outlet for the second conduit, each channelhaving a larger cross section than that of a groove; the arrangementbeing such that if, in use, one fluid is passed through the firstconduit with the pulsatile flow, vortex mixing occurs in the dimples inthe membranes; characterized in that the maximum external diameter ofeach dimple, in a plane substantially parallel to the plane of thecorresponding membrane sheet is, less than the corresponding diameter ofthe corresponding plate depression.
 17. An assembly according to claim16, in which the channels extend one along each longer side of eachplate, with a plurality of transverse channels interconnecting the sidechannels to provide a ladder-like array of channels.